A Przewalski's horse, the only truly wild and untamed horse known today, is shown in Khomyntal, western Mongolia. Its genes have been studied and showed no detectable mixture of genes from any other breeds.

Photo: Claudia Feh, Associated Press

A Przewalski's horse, the only truly wild and untamed horse known...

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In this undated photo provided by Ludovic Orlando via Nature, two pieces of a 700,000-year-old horse metapodial bone, just before being extracted for ancient DNA, are shown. From a tiny fossil bone found in the frozen Yukon, scientists have deciphered the genetic code of an ancient horse about 700,000 years old _ nearly 10 times older than any other animal that has had its genome mapped. The work was published Wednesday, June 26, 2013, by the journal Nature and discussed at a science conference in Helsinki. (AP Photo/Ludovic Orlando via Nature)

The frozen toe bone of a horse that died in a Canadian snowdrift 700,000 years ago has yielded unique insights into equine evolution, from the animal's ancestors 4 million years ago to its varied relatives of today - the wild ones and the purebreds, the zebras, and even the humble donkeys.

Beth Shapiro, an evolutionary biologist at UC Santa Cruz, has roamed the frozen gold fields of the Klondike for years, collecting ancient bones of all kinds to analyze their genes, and her collection includes bones of bison, caribou, the wooly mammoth and ancient horses.

Now she and Mathias Stiller, a postdoctoral fellow in her lab, have joined 60 other international scientists to report they have reconstructed the genome of ancient horses to reveal a remarkable story of equine populations that rose and fell under the influence of climate changes as the world's ice ages waxed and waned.

The research group, headed by Ludovic Orlando and Eske Willerslev of the Natural Museum of Denmark, is publishing a report on its findings Thursday in the journal Nature.

Dating the toe bone

The horse bone, dug out of the solid permafrost in the Yukon Territory, was dated from a layer of ash remaining from a volcanic eruption about 800,000 years ago. The ash could be dated by the magnetism still held inside its rocks, Shapiro said.

To solidify the project, Shapiro and Stiller determined the sequence of genes they found in other ancient bones. The tiny cellular organs in those bones, called mitochondria, are passed through the female lineage and are used like molecular clocks to estimate the age of the animals.

By analyzing the gene sequences of other members of the Equus lineage - horse bones 43,000 years old plus five modern horses as well as zebras and donkeys, the scientists now have the longest genetic history ever recorded for the evolution of a living organism - from microbes to humans.

Shapiro brought the story to familiar ground. "We can see that the horses of North America were most numerous 30,000 years ago, but their population gradually declined as the ice age retreated until they actually became extinct on this continent about 12,000 years ago," she said in an interview. "The horses only recovered after the Europeans imported them.

"We can now understand too how population changes long before that time reflected the climate changes of past ice ages," she added.

Truly wild horse

The research team also analyzed genes of the only truly wild and untamed horse known today. It is named Equus ferus przewalskii. A rare Mongolian animal called Przewalski's horse lives today in a few zoos worldwide and as wild herds in a few Asian national parks. Their genome is now part of the horse genetic database.

Until now the 50,000-year-old genome of the Denisovans, a primitive human group that lived in Siberia near the Neanderthals, was the oldest gene sequence ever resolved.

The project's scientists noted that DNA in all genomes normally degrades so rapidly that truly ancient genes cannot be identified. But the Yukon Territory's temperatures are so cold all year that the Thistle Creek gold fields in the Klondike, where the bones were found, can apparently preserve DNA indefinitely, Shapiro said.

Scientists had previously believed that the most remote common ancestor of horses and their related animals dated back about 2 million years, but in their report, the Equus team estimated that the horse lineage existed at least twice as far back.

James Cahill, a doctoral student in Shapiro's lab, studied the genes of Mongolia's famed Przewalski's horse for the project, and helped determine that the wild horse is truly wild - it showed no detectable mixture of genes from any other breeds, Shapiro said.